This invention relates to the measurement of operating parameters of DC motors. More particularly, the invention relates to a method of determining the inductance of inductive elements driven by an AC to DC converter.
When a new motor is commissioned, it is highly desirable to establish the baseline characteristics of the motor. In this regard, the operational armature inductance (i.e., the inductance of the armature when the associated field winding is energized to an operational current level) is of particular importance. As is well known, the inductance of the motor armature decreases with increasing field current. As a result, the armature inductance level with no field current can be 100-200% greater than the saturated armature inductance level. Pre-commission armature inductance measurement conducted without a field current is therefore inadequate for use in tuning motors having high saturation characteristics.
Accordingly, armature inductance must be determined during commissioning with the associated field current at an operational level. In order to avoid shaft movement, which may cause overspeed or damage to load equipment, the usual method for determining operational armature inductance requires that the motor shaft be locked in place. The operational field current is then established and a step current applied to the armature. The armature inductance is determined based on the response of the actual armature current to the step input.
The above-described method of determining armature inductance can be highly time consuming because it requires that the motor shaft be locked and it requires that trending of various step current responses be carried out. With some motors and installations, the inductance measurement task can increase commissioning time by as much as a day.
There is accordingly a need for a method of determining an operational armature inductance value for a converter-driven DC motor that does not require that the shaft of the motor be locked in place.
A method for determining an inductance value of an inductive element is provided that includes energizing the inductive element using an N-pulse AC to DC converter in electrical communication with the inductive element and an AC source. The method further includes determining at each of a plurality of periodic time intervals an inductive element voltage value, an inductive element current value, and an equivalent source phase angle. An Nth harmonic impedance squared value is then determined for the inductive element using the inductive element voltage and current values and the equivalent source phase angles. The inductive element inductance value is then calculated using the Nth harmonic impedance squared value and an AC source frequency value.